feat(strategy): M8.3 策略模型 + /api/strategies（命名配置资产，可展开为 ResearchSpec）

- StrategyDefinition：universe/factors/selection/rebalance/costs/portfolio +
  price_adjustment（除 period 外完整策略定义）；to_research_spec(period) 展开为标准 Spec
- strategy 表（migration e1f2a3b4c5d6，MySQL 已应用；name 唯一）+ StrategyRepository
- /api/strategies：POST/GET/DELETE + POST /{id}/expand（period+initial_capital → ResearchSpec）
- tests/test_strategies.py（repo CRUD/同名/expand、API CRUD/400/404）；全量 pytest 通过

backend/app/infrastructure/persistence/migrations/versions/e1f2a3b4c5d6_strategy_table.py

@@ -0,0 +1,38 @@
+"""strategy 表（M8.3 策略持久化）
+
+Revision ID: e1f2a3b4c5d6
+Revises: d8e0b2f3c4d5
+Create Date: 2026-09-09
+
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "e1f2a3b4c5d6"
+down_revision: str | None = "d8e0b2f3c4d5"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "strategy",
+        sa.Column("id", sa.String(length=32), nullable=False),
+        sa.Column("name", sa.String(length=64), nullable=False),
+        sa.Column("description", sa.String(length=300), nullable=False),
+        sa.Column("spec_type", sa.String(length=16), nullable=False),
+        sa.Column("config_json", sa.Text(), nullable=False),
+        sa.Column("version", sa.String(length=16), nullable=False),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("name", name="uq_strategy_name"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("strategy")